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Technical Topics => Part 15 AM and FM Station Operation => Topic started by: Brian on December 31, 2020, 1422 UTC

Title: Helical antenna
Post by: Brian on December 31, 2020, 1422 UTC
Not strictly part 15 but I can't think of a better place to put this question. I'm thinking of constructing an antenna similar to a Valcom helical antenna for MW. I can get a fiberglass pole from Spiderbeam.https://www.spiderbeam.com/index.php?cat=c2_Fiberglass%20Poles.html&XTCsid=a2ffd96b7e87e34e64b1c65f54310ffb They come in various lengths but I think 18mt (60 feet) is about the max I can handle (financially and physically).
Has anybody here had experience in constructing these type of aerials? Any tips appreciated.

Oh,  a happy new year to all here.  :)
Title: Re: Helical antenna? HAM Talk Live talks about, well, something like that.
Post by: ThaDood on January 02, 2021, 0528 UTC
Huh... Interesting timing, since the latest HAM Talk Live is about constructing a helical antenna,    https://www.spreaker.com/user/hamtalklive/episode-240-new-helical-antennas        Now, for Part #15 MW broadcasting, you have that 3 meter length limit, but they don't say how wide that should be. (Hint-hint...)
Title: Re: Helical antenna
Post by: RobRich on January 02, 2021, 1253 UTC
If disregarding part 15 limits....

Curious, how much horizontal space do you have? I ask because you might find a T-antenna of interest also. Up 60' then out 60' on each side for example. Basically a top-loaded vertical with the "top-hat" increasing efficiency.

Alternatively, you might even try a vertical helical with top loading, too. I am not sure it would be any more efficient, but I suppose it might be somewhat easier to match depending on how low in frequency you intend to go. YMMV.

Either way you should be considering the other "half" of the antenna, too. As in the radial field if ground mounted, or you might even try helically wound radials if in the air. :)
Title: Re: Helical antenna
Post by: Brian on January 03, 2021, 2205 UTC
I think the pole is too narrow in diameter for a helical. May just go for a top loaded vertical instead. The inv L I have up works pretty well. A T isn't practical really as the vertical section would not be close enough to my existing ground system.